Studies are being conducted on the biosynthesis of thyroxine through an investigation of the action of purified thyroid peroxidase (and other peroxidases) on the iodination of tyrosine-containing oligopetides. Also, the release of diiodotyrosine and thyroxine from oligopeptides containing these residues through the action of cathepsin D is being studied in relation to the specificity and mode of action of this enzyme.